1. Field of the Invention
This invention relates to adhesive and sealant compositions having improved thermal conductivity and strength properties at elevated temperatures and resistance to thermal degradation. The compositions have particular utility for mounting heat generating electrical components to heat sinks and will be described in connection with such utility, although other uses such as structural bonding and as machinable filling are contemplated.
2. Prior Art
The increasing density of components on printed circuit assemblies and the use of higher-power components often necessitates the use of board or chassis mounted heat sinks. A heat sink is a thermally conductive component, usually metal, designed to radiate or conduct unwanted heat away from a heat generating electronic component in order to maintain the electronic component below a maximum acceptable operating temperature. A heat sink may be considered as a complete thermal system consisting of the heat generator and its mounting hardware, thermally conductive interfacing materials to bring the mounting surface of the component into intimate contact with a surface of the sink, and the thermal dissipater itself. For practical purposes the thermally conductive interfacing material generally is the weakest (thermal) link in a heat sink system.
Various thermally conductive interfacing materials are known in the art and are available commercially. Each has its advantages and disadvantages. One such type of prior art thermally conductive interfacing material comprises metal- or metal oxide-filled epoxy resin. Metal- or metal oxide-filled epoxy resins provide adequate coefficient of thermal conductivity (K=1.255 W/m .degree.C.), good electrical insulation and high adhesive strength. Metal- or metal oxide-filled epoxy resins are available as two-component systems exhibiting relatively short working pot life or as one component systems requiring five to seven days at room temperature to cure, or they may be cured in one to three hours by heating to elevated temperature. However, heating at elevated temperature may damage heat sensitive electronic components. Moreover, metal- or metal oxide-filled epoxy resins shrink somewhat during cure. Accordingly, a buffer material must be placed around fragile electrical components to prevent fracturing from shrinkage. However, the imposition of a buffer material may adversely affect thermal conductivity characteristics.
Silicone greases also have been employed by the prior art as thermally conductive interfacing materials. Silicone greases provide coefficients of thermal conductivity of K=0.20 to 0.42 W/m .degree.C., and good electrical insulation. Silicone greases are able to sustain prolonged high temperature exposure which makes silicone resins the presently preferred thermal interfacing materials for use with high-heat dissipating components such as power transistors. However, silicone greases are relatively thick, making them somewhat difficult to apply. Moreover, silicone greases are not adhesives, and thus separate mechanical mounting hardware is required for securing the components to the sink. Additionally, silicone greases are known to migrate into electrical contacts such as switches, plug-in connectors, relay contacts and variable resistors. Also, silicone greases do not cure and thus may collect dirt which can contain metal and other conductive particles which could cause shorts.
As this is apparent from the above discussion, none of the currently available thermally conductive interfacing materials are completely satisfactory since all prior art thermally conductive interfacing materials at best are a tradeoff of preferred application characteristics, (i.e. processing speed, ease of application, pot life and cure conditions), preferred physical characteristics, (i.e. thermal conductivity characteristics and electrical properties and adhesive strength), and repairability.
It is thus a primary object of the present invention to provide an improved thermally conductive interfacing system, i.e. processes, materials and products which overcome the aforesaid and other disadvantages of the prior art.
Other objects of the present invention are to provide improved thermally conductive adhesive compositions for interfacing high-heat dissipating components and the like with heat sinks and the like which compositions demonstrate relatively high coefficients of thermal conductivity, extended pot life and ease of application, rapid dry-to-touch cure and dry fillets, good electrical properties, good adhesive strength at elevated temperatures, and ease of repairability. Yet other objects will in part appear obvious and will in part appear hereafter.
The invention accordingly comprises the processes involving the several steps and relative order of two or more of such steps with respect to each other, and the devices, materials, and compositions possessing the several features, properties and relations of elements which are exemplified in the following disclosure and scope of application of which will be indicated in the claims.